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http://www.diva-portal.org
This is the published version of a paper presented at The 28th
International Conference on Efficiency,Cost, Optimization,
Simulation and Environmental Impact of Eeergy Systems,June 29 -
july 3, 2015, Pau,France.
Citation for the original published paper:
Baldi, F., Ahlgren, F., Nguyen, T., Gabrielii, C., Andersson, K.
(2015)
Energy and exergy analysis of a cruise ship.
In: Proceedings of ECOS 2015 - the 28th International Conference
on Efficiency, Cost,
Optimization, Simulation and Environmental Impact of
Energy Systems
N.B. When citing this work, cite the original published
paper.
Permanent link to this
version:http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-45753
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PROCEEDINGS OF ECOS 2015 - THE 28TH INTERNATIONAL CONFERENCE
ON
EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL
IMPACT OF ENERGY SYSTEMS
JUNE 30-JULY 3, 2015, PAU, FRANCE
Energy and exergy analysis of a cruise ship
Francesco Baldia, Fredrik Ahlgrenb, Tuong-Van Nguyenc , Cecilia
Gabrieliid and
Karin Anderssone
a Department of Shipping and Marine Technology, Chalmers
University of Technology, Gothenburg,
Sweden. [email protected] b Kalmar Maritime Academy,
Linnaeus University, Kalmar, Sweden. [email protected] ,
c Department of Mechanical Engineering, Technical University of
Denmark, Lyngby, Denmark.
[email protected] d Department of Shipping and Marine Technology,
Chalmers University of Technology, Gothenburg,
Sweden. [email protected] d Department of Shipping
and Marine Technology, Chalmers University of Technology,
Gothenburg,
Sweden. [email protected]
Abstract:
The shipping sector is today facing numerous challenges. Fuel
prices are expected to increase in the medium-long term, and a
sharp turn in environmental regulations will require several
companies to switch to more expensive distillate fuels. In this
context, passenger ships represent a small but increasing share of
the industry. The complexity of the energy system of a ship where
the energy required by propulsion is no longer the trivial main
contributor to the whole energy use thus makes this kind of ship of
particular interest for the analysis of how energy is converted
from its original form to its final use on board. To illustrate
this, we performed an analysis of the energy and exergy flow rates
of a cruise ship sailing in the Baltic Sea based on a combination
of available measurements from ship operations and of mechanistic
knowledge of the system. The energy analysis allows identifying
propulsion as the main energy user (41% of the total) followed by
heat (34%) and electric power (25%) generation; the exergy analysis
allowed instead identifying the main inefficiencies of the system:
exergy is primarily destroyed in all processes involving combustion
(88% of the exergy destruction is generated in the Diesel engines
and in the oil-fired boilers) and in the sea water cooler (5.4%);
the main exergy losses happen instead in the exhaust gas, mostly
from the main engines (67% of total losses) and particularly from
those not equipped with heat recovery devices. The improved
understanding which derives from the results of the energy and
exergy analysis can be used as a guidance to identify where
improvements of the systems should be directed.
Keywords:
Energy analysis; exergy analysis; low carbon shipping
1. Introduction
1.1 Background
According to the third IMO GHG Study 2013, in 2012 shipping
contributed to global anthropogenic
CO2 emissions with a total of 949 million tonnes, which
represents roughly the 2.7% of the total
[1]. Although such contribution appears relatively low, the
trend is that shipping will play an even
greater role in the CO2 emissions in a near future due to the
increased transport demand according
to all IMO future scenarios. As an example, global transport
demand has increased by 3.8 % in
2013, compared to a global GDP growth of 2.3 % the same year,
which shows how shipping tends
to rise even faster than global economy [2].
International Energy Agency data from marine bunker show that
the OECD countries in fact have
reduced the CO2 impact from shipping, but a larger amount has
been moved to the non-OECD
countries [3]. The fact that shipping needs to even further
reduce its CO2 emissions in the near
future is essential for being able to achieve the goals of
maintaining global average temperature
increase below 2oC by 2050 [4]. Finally, in the Baltic Sea an
emission control area is enforced by
the International Maritime Organisation since January 2015 which
stipulates that the fuel used must
not contain more than 0.1 % sulphur, therefore requiring the use
of more expensive distillate fuels.
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Altogether, these conditions present a challenge to shipping
companies, which are attempting to
reduce their fuel use in an attempt to reduce both environmental
impact and operative costs. A wide
range of fuel saving solutions for shipping are available and
partially implemented in the existing
fleet, both from the design and operational perspective; several
specific studies have been
conducted on these technologies, and a more detailed treatise
would be out of the scope of this
work. In this context, it has been acknowledged that the world
fleet is heterogeneous, and measures
need to be evaluated on a ship-to-ship basis [5,6]. In this
process, a deeper understanding of energy
use on board of the specific ship is vital.
1.2 Previous work
A number of studies concerning ship energy systems can be found
in literature. Thomas et al. [7]
and Basurko et al. [8] worked on energy auditing fishing
vessels; Shi et al. [9] proposed models for
predicting ship fuel consumption for some specific vessel types;
Balaji and Yaakob [10] analysed
ship heat availability for use in ballast water treatment
technologies. These studies have been of
particular interest in their relative fields, but a more
comprehensive approach of the totality of the
ship energy system is missing. In addition, an analysis purely
based on the First law of
thermodynamics does not account for the irreversibilities of the
systems and for the different quality
of heat flows [11]. Exergy analysis, which is based on both the
First and the Second laws of
thermodynamics, can help addressing this shortcoming. Widely
used in other industrial sectors,
exergy analysis in not commonly employed in maritime technology
studies, and is mostly related to
waste heat recovery systems [12,13] and refrigeration plants
[14,15]. The application of exergy
analysis in shipping is still limited; Zaili and Zhaofeng [16]
proposed the energy and exergy
analysis of the propulsion system of an existing vessel showing
that there is potential in improving
ship power plant efficiency by recovering the exergy in the
exhaust gas and by improving
operations of the main engines.
In a previous study of the energy and exergy analysis of a
product tanker [17] the dominance of
propulsion as main energy user on board was highlighted,
together with the substantial availability
of waste heat for recover. On cruise vessels, the number of
different uses of energy is larger and a
complex system of different energy carriers (chemical, thermal,
electrical or mechanical) is present
in order to fulfil the needs for transport combined with
passenger services and comfort, such as
cooking and cooling in restaurants, air conditioning, and
passenger entertainment facilities.
The complexity of the energy system of a ship where the energy
required by propulsion is no longer
the main contributor to the whole energy use thus makes this
kind of system of particular interest
for the analysis of how energy is transformed used on board. The
complexity of such systems was
modelled and investigated previously by Marty et al. [18,19],
but to the best of our knowledge there
is no study in literature describing cruise ships’ energy and
exergy analyses based on actual
measurements.
1.3. Aim
The aim of this paper is to provide a better understanding of
how energy and exergy are used on
board of a cruise ship and where the largest potential for
improvement is located by applying energy
and exergy analysis to the a case study. The combination of a
method rarely applied in the shipping
sector to a ship type featuring a complex energy system is
considered as the main contribution of
this work to the existing literature in the field.
2. Methodology This paper proposes the application of energy and
exergy analysis (further described in Section 2.1)
as a mean for improving understanding of energy conversion on
board of a cruise ship. This
application is shown for a specific case study vessel (see
Section 2.2) for which extensive
measurements from on board logging systems were available (see
Section 2.3 for details on data
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gathering and processing). The results from the energy and
exergy analyses are then discussed in
order to propose possible improvements for ship operations and
design.
2.1 Energy and exergy analysis
Energy and exergy analysis were performed on the system of
study, taking the ship energy system,
as presented above, as control volume. The energy flow rates are
calculated by assuming that the
chemical energy of the fuel flows is equal to its lower heating
value, and the physical energy is
taken as its relative enthalpy. The reference state, both for
energy and exergy analysis, is assumed
to be at atmospheric pressure and at the measured sea water
temperature. Such an analysis allows
tracking all the energy streams flowing through the ship and
depicts the main heat and power users.
Energy may be transformed from one form to another, but it can
neither be created nor destroyed;
this results in the fact that a conventional energy analysis
provides limited information on the
system inefficiencies. Exergy is defined as the `maximum
theoretical useful work as the system is
brought into complete thermodynamic equilibrium with the
thermodynamic environment while the
system interacts with it only’. At the difference of energy,
exergy is not conserved in real processes,
and the exergy destroyed, or irreversibility rate, quantifies
the system irreversibilities. The general
exergy balance can be written as:
in outI B B , (1) where:
I denotes the irreversibility rate, also called exergy
destruction, which can also be calculated
from the Gouy-Stodola theorem;
inB is the exergy flow rate entering the component/system under
investigation;
outB represents the corresponding exergy outflow. This term is
normally further subdivided in
two parts: products( prodEX ) and losses ( lossEX )
The exergy of a material flow is divisible into its physical,
chemical, kinetic and potential
components, in the absence of nuclear and magnetic interactions.
The physical exergy represents
the maximum amount of work obtainable from bringing the material
stream from its initial state to
the environmental state, defined by p0 and T0, taken here as the
ambient pressure and the seawater
temperature. The chemical exergy represents the maximum amount
of work obtainable as the
stream under consideration is brought to the dead state, by
chemical reaction and transfer processes.
The fuel chemical exergy is assumed equal to its higher heating
value, which is derived based on
the fuel H/C ratio according to the equation proposed by Szargut
et al. [20]. The potential and
kinetic exergies are neglected. The exergy transferred with
power has the same value as its energy,
while the exergy transferred with heat is lower and its value
depends on the temperature at which
heat transfer takes place. For more details, the reader is
referred to the reference books of Szargut et
al. [20], Kotas [21] and Moran [22].
The system performance is measured using several performance
indicators:
the exergy efficiency ( t ), defined as the ratio between the
product pB and the input inB exergies:
p
t
in
B
B (2)
where the product exergy represents the desired output of the
component or system, and the
input exergy denotes the resources required to drive this
process.
the exergy loss ratio ( ), proposed in [23], which illustrates
how much of the exergy input to the system is actually destroyed
through irreversibilities:
in
I
B (3)
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the irreversibility share ( ), proposed in the works of Kotas
[21] and Tsatsaronis [24], which is
defined as the ratio between the exergy destroyed in the i-th
component iI in relation to the total
system irreversibilities totI .
ii
tot
I
I (4)
2.2 Case study vessel The ship under study is a cruise ship
operating on a daily basis in the Baltic Sea between Stockholm
(Sweden) and Mariehamn (the Åland islands). The ship is 176.9 m
long and has a beam of 28.6 m,
and has a design speed of 21 knots. The ship was built in Aker
Finnyards, Raumo Finland in 2004.
The ship has a capacity of 1800 passengers and has several
restaurants, night clubs and bars, as well
as saunas and pools. This means that the energy system regarding
the heat and electricity demand is
more complex than a regular cargo vessel in the same size.
Typical ship operations, although they
can vary slightly between different days, are represented in
Figure 1. It should be noted that the ship
stops and drifts in open sea during night hours before mooring
at its destination in the morning, if allowed by weather
conditions.
Fig. 1. Typical operational profile (ship speed, main engines
load and auxiliary engines load) for
the selected ship.
The ship systems are summarized in Figure 2. The propulsion
system is composed of two equal
propulsion lines, each made of two engines, a gearbox, and a
propeller. The main engines are four
Wärtsilä 4-stroke Diesel engines (ME) rated 5850 kW each. All
engines are equipped with selective
catalytic reactors (SCR) for NOX emissions abatement. Propulsion
power is needed whenever the
ship is sailing; however, it should be noted that the ship
rarely sails at full speed, and most of the
time it only needs one or two engines operated simultaneously.
When only one engine is used,
power can be delivered to only one propeller. This requires the
use of a significant rudder angle for
keeping a straight course, which in turn increases ship
resistance and, consequently, the amount of
power to be delivered to the propeller.
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Auxiliary power is provided by four auxiliary engines (AE) rated
2760 kW each. Auxiliary power is
needed on board for a number of alternative functions, from
pumps in the engine room to lights,
restaurants, ventilation and entertainment for the
passengers.
Auxiliary heat needs are fulfilled by the heat recovery steam
generators (HRSG) located on the
exhaust pipes of all four AEs and on two of the four MEs, and by
the heat recovery on the HT
cooling water systems (HRHT). When the available waste heat from
the engines is not sufficient for
fulfilling total heat demand, two oil-fired auxiliary boilers
(AB) can be used to provide the
remaining amount of heat. This situation mainly occurs when the
ship is berthed in port, or during
winter. The heat is needed for passenger and crew accommodation,
as well as for the heating of the
highly viscous heavy fuel oil used for engines and boilers. This
last part, however, is drastically
reduced since the 1st of January 2015, as new regulations
entering into force require the use of low-
sulphur fuels, which require a much more limited heating.
Fig. 2. Schematic representation of ship energy systems
2.3 Data gathering and processing
The ship under study is equipped with an extensive system for
measuring and logging of
operational variables, which logs the data with a 60 second
interval. For this study an averaged 15
minute interval was chosen in order to cover a total of
approximately one year of ship operations
under the constraints related to the maximum number of data
points in the database export tool.
A detailed accounting of all relationships and assumptions
employed in this study in order to
process the raw measured data are shown in Table A1 in Appendix
A. Hereafter only the most
relevant assumptions are discussed.
Main engines power and fuel mass flow ( ,fuel MEm ) were not
directly measured. In this study it was
assumed that measures of the normalized fuel rack position
(frpnorm) can be used as a predictor for
the amount of fuel injected per cycle. The fuel flow to the main
engines is consequently calculated
according to the following equation:
, , , 0 1,
MEfuel ME fuel ME des norm
ME des
nm m a a frp
n
, (5)
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where the subscript des refers to design conditions of the
engine at 100% of the maximum
continuous rating (MCR) and nME represents the main engine
speed. The regression coefficients a0
and a1 where determined based on the engine shop trial tests
documents. Estimations calculated
using (5) were validated against fuel flow measurements obtained
from a recently installed mass
flow meter. MEs delivered power ( MEW ) was then calculated
according to (6):
,63.6fuel ME
ME
ME
mW
bsfc (6)
where the break specific fuel consumption (bsfc) of the MEs was
calculated using a 2nd degree
polynomial function calibrated on shop trial data. This method
for the estimation of the required
propulsion power involves certain accuracies, mostly related to
engine bsfc (a margin of 5% is
generally considered in related ISO standards) and to the
relationship between fuel mass flow,
engine speed and fuel rack position. However, given that no
direct measurement of propeller power
was available, this method is believed to be far more accurate
than numerical estimations of the
required propulsive power based on ship particulars and vessel
speed.
The AEs power was available from on board measurements. The fuel
mass flow rate to the AEs was
calculated solving (6) for the fuel mass flow rate, where AEs’
bsfc was estimated according to the
same principle as described for the MEs.
The heat demand was not directly measured, and therefore needed
to be estimated. As previously
mentioned, on board heat demand is fulfilled by three different
systems: the HRSGs, the HRHT and
the ABs. The heat recovered in the HRSGs was estimated based on
calculated engine exhaust flow
and measured temperatures before and after the HRSGs; no
information was available regarding the
heat recovered in the HRHT; finally, the AB daily fuel
consumption was available from a second
logging system. In order to provide a reasonable assumption for
the contribution of each of the
above mentioned systems to the total amount of heat generated on
board, it was assumed that heat
demand is constant during each day, as suggested by Marty [19] .
In addition, discussions with the
crew allowed making the assumption that the ABs are only used
when the main engines are not
running. Based on these considerations, the following
approximation was employed in this study:
,,( )
( ) 500HRSG port i ifuel AB i
tot fuel AB
port port
Q t tmQ t LHV
t t
[kW], (7)
where the first term represents the contribution from the oil
fired boilers ( ,fuel ABm , portt , and AB
represent the mass of fuel consumed by the ABs during the port
stay, the time spent by the ship in
port and the energy efficiency of the ABs respectively ), the
second term that of the HRSGs
operated in port ( ,HRSG portQ and it represent the heat flow
recovered during port stays and the i-th
time interval in which the port stay is divided respectively),
and the third that of the HT cooling
systems based on design calculations provided by the shipyard.
When the ship is
sailing/manoeuvring the ABs are turned off, and the required
heat on board is generated by the
HRSGs and the HRHT:
, ( ) ( ) ( )rec HT tot HRSGQ t Q t Q t (8)
where the total heat demand ( )totQ t was calculated using (7)
and the heat flow available from the
HRSG ( )HRSGQ t is calculated based on available measurements of
exhaust temperature and on the
calculated exhaust gas flow rate from the engines.
3. Results
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3.1 Operational profile
Fig. 3. Time spent by the ship in different operational
modes
(a) (b)
Fig. 4. Load distribution for a) main engines and b) auxiliary
engines
As shown in Figure 3 the ship spends most of the time sailing,
while a significant amount of time is
also spent in port. This is not surprising, in relation with the
typical operations of this type of ship
where loading and unloading of passengers is an operation that
require a significant amount of time.
Figure 4 shows the load distribution for the main engines (a)
and auxiliary engines (b), respectively.
As it is observable from the figure, the main engines are most
often operated at very low load,
which leads to sub-optimal conditions in terms of efficiency and
wear. This is a result of two
concurring factors, as discussions with the crew revealed:
The ship is operated most of the time at a speed which is much
lower than the design value. This leads consequently to a strong
reduction in propulsion power demand
The engines are divided in two groups, each driving one
propeller. This means that, even at very low load, it is not
possible to operate on only one engine at medium-high load.
3.2 Energy analysis
Figure 4 shows the Sankey diagram for ship prime movers,
converters, and users, where values are
presented numerically in Table A2 in Appendix A. From the users’
side, it can be seen that the
energy demand for auxiliary power is comparable in size to that
for propulsion (see Table 1). This is
situation is expectable in the case of cruise ships / passenger
ferries, but not common in other ship
types. Thrusters, although they represent a high punctual
demand, do not significantly contribute to
34%
7%
59%
Port Stay
Maneuvring
Sea Going
0%
2%
4%
6%
8%
10%
12%
0% 10% 20% 30% 40% 50% 60% 70% 80%
Freq
uen
cy o
f o
ccu
rran
ce
Engine load
0%
2%
4%
6%
8%
10%
12%
14%
16%
0% 10% 20% 30% 40% 50% 60% 70% 80%
Freq
uen
cy o
f o
ccu
rran
ce
Engine load
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the overall energy use. Auxiliary heat demand is also
particularly large, but is mostly fulfilled by
heat recovery boilers. It should be noted, however, that the
yearly fuel consumption from the
auxiliary boilers in the case under study is significant (5.2
%).
The contribution from the HT cooling water systems is also
significant, comparable to that of the
boilers and the HRSGs. Although this value was not directly
measured and is therefore subject to a
larger uncertainty, this observation suggests that heat
integration has been carefully and
successfully taken into account in the design of this particular
vessel.
The energy analysis also shows that a large amount of heat is
rejected to the environment, mainly
with the exhaust gases exiting the heat recovery steam
generators installed after the main engines,
and heat from the low-temperature and seawater cooling systems.
The amount of energy dispersed
to the environment is in the same order of magnitude as the heat
demand of the whole ship energy
system. This situation suggests that additional heat could be
harvested for other uses on-board, e.g.
for use in heat recovery and heat-to-power systems, which would
result in a smaller fuel
consumption of the boilers or/and of the engines. This aspect
will be however further investigated
using the exergy analysis, which gives a better picture of
energy quality and a better estimation of
the amount of energy that could be actually recovered and
converted into electricity. Large amount
of energy is also dispersed via the LO cooling. This waste heat
flow is by no means recovered on
ship systems, differently from the heat to the HT systems.
Figure 5 shows the repartition of the energy use among different
users and for the different
operational modes. Propulsion represents the main part of energy
use, but is only present when at
sea or manoeuvring. Electric energy demand is instead rather
constant over time and therefore it
scales proportionally to the time in each phase. It should be
noted that, both in Figure 1 and in
Tables 2 and 3 the category “port stays” also includes the time
spent by the ship drifting at sea.
The absence of any dedicated measurement made it impossible to
identify the individual users;
however, the use of bow thrusters during manoeuvring constitute
a clear spike in the total auxiliary
power demand and are therefore possible to separate from the
total.
Fig. 4. Sankey diagram for ship energy systems. Values represent
the aggregated figures over one
year of operations. All main engines and auxiliary engines are
grouped together.
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Fig. 5. Yearly energy demand for different users, separated per
operational mode.
Table 1: Yearly shares for the different energy users on board,
divided by operational mode
User Port stay Manoeuvring Sea going All modes
Propulsion 0,0% 1,7% 39,6% 41,3%
Thrusters 0,0% 0,3% 0,0% 0,3%
Other electric power users 8,3% 1,2% 15,3% 24,8%
Fuel heating 0,4% 0,1% 1,2% 1,7%
Other heat users 11,0% 2,0% 18,9% 31,9%
All users 19,7% 5,3% 75,0% 100,0%
3.3 Exergy analysis
The observation of the Grassmann diagram (Figure 6) allows for
identifying, locating and
quantifying the main sources of exergy losses and destruction.
Numerical results are presented in
Tables 2 and 3 in the text and Table 2A in the appendix. Most
exergy destruction takes place in the
main and auxiliary engines (respectively 53.1% and 27.8% of the
total) followed by the boilers
(6.9%), where the high rate of exergy destruction is strongly
connected to the process of conversion
of chemical to thermal energy, as well as to mixing and friction
phenomena and heat transfer. A
significant part of the exergy destruction takes place in the
cooling systems (9.1%), which
highlights potential for improvement in the design of the heat
exchanger network. Finally the
exergy flow rate lost to the environment in the exhaust gas
after the HRSGs (8.0% of the total
exergy input, 20.3% if compared to the total exergy output of
the system) also represents a
significant potential for improvement. It should be noted, as
shown in Table 3, that most exergy
losses take place during the seagoing phase, when on board heat
demand is fulfilled through the use
of waste heat available on board. The recoverable exergy
theoretically available during port stays
constitutes however more than half (52.5%) of the total exergy
flow rate produced by the boilers.
The analysis of exergy performance indicators further highlights
a number of observations about the
system. Main engines show a lower efficiency compared to the
auxiliary engines (34.7% compared
to 38.3%) despite their higher efficiency at design conditions
(46.8% compared to 44.2%), which
provide further evidence to the fact that the main engines are
often operated in non-optimal
conditions. The high irreversibility ratio of the boilers
(70.8%) suggests that reducing their use
should be a priority in view of the exergy optimization of the
system. Among the heat exchangers,
the highest potential for improvements appears to lie in the
lubricating oil coolers and in the jacket
water coolers, where the high temperature difference between hot
and cold flows suggests that the
heat exchange process could be improved.
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
Propulsion Thrusters Other el.consumers
Fuelheating
Other heatconsumers
Shar
e o
f to
tal e
ner
gy u
se
Port stay
Maneovring
Sea going
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Fig. 6: Grassmann diagram for ship energy systems. Values
represent the aggregated figure over
one year of operations. All main engines and auxiliary engines
are grouped together.
Table 2: Exergy efficiency, irreversibility ratio and
irreversibility share for ship energy system
thermal components
Component Exergy efficiency Exergy loss ratio Irreversibility
share
Main engines 34,7% 46,2% 53,1%
Auxiliary engines 38,3% 44,0% 27,8%
Auxiliary Boilers 29,2% 70,8% 6,9%
Charge air cooler (ME) - 4,7% 0,5%
Charge air cooler (AE) - 3,2% 0,2%
Lub oil cooler (ME) - 6,6% 0,6%
Lub oil cooler (AE) - 10,5% 0,5%
Jacket water cooler (ME) - 7,8% 1,0%
Jacket water cooler (AE) - 7,4% 0,5%
HT - LT mixer - 1,7% 0,4%
LT - SW cooler - 65,7% 5,4%
HRSG (ME) 37,0% 18,8% 1,5%
HRSG (AE) 28,5% 15,9% 1,4%
Table 3: Yearly shares for the different exergy losses on board,
divided by operational mode
Flow Port stay Manoeuvring Sea going All modes
Exhaust, AE 9,1% 1,7% 19,2% 30,0%
Exhaust, ME 0,0% 2,6% 64,5% 67,1%
LT - SW cooler 0,6% 0,2% 2,2% 2,9%
All exergy losses 9,7% 4,5% 85,9% 100,0%
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4. Discussion The energy and exergy analysis of the selected
ship reveal a rather well thought design, where there
has been a significant attempt into the reduction of energy use,
especially from the heat demand
perspective. The amount of energy recovered from the exhaust gas
and the engine cooling systems
amounts to the most significant fraction of the overall head
demand on board, and the boiler is used
only in those situations when on board generated waste heat
would not be sufficient to fulfil the
totality of the heat demand. However, the system shows
possibilities of improvement.
4.1 Suggested improvements
From an operational point of view, the main engines are often
operated at very limited load, which
significantly reduces the efficiency of the energy conversion.
This situation is mostly due to high
installed power (the vessel was designed for 21 knots but is
normally operated at a maximum of 16
knots and, most often, at even lower speeds). In some parts of
the journey operations on one engine
only are prevented by regulations that impose to use both
propeller lines in order to ensure the
require capability to manoeuvre; it should be noted, however,
that the achieved higher engine
efficiency might be compensated by higher hull rudder
resistance.
From a retrofitting/design perspective, even if engine
substitution might not be possible due to the
high related investment cost, engine de-rating could be a viable
option for improving the off-design
efficiency of the main engines. This could be achieved through
cylinder disconnection, which
would allow operating fewer cylinders closer to design
conditions, and substitution of the existing
turbocharger with one designed for lower power. An alternative
worth investigating could also be
that of an hybridisation of the whole system, through the
installation of shaft motors/generators,
which would allow both main and auxiliary engines to contribute
to both propulsive and auxiliary
electric power demand and, therefore, increase the flexibility
of the system.
Efforts for improving the performance of a ship energy system
should however not only focus on
the main engines but on avoidable irreversibilities in the rest
of the system, such as those caused in
the HRSGs and cooling systems. These may be reduced by
decreasing the temperature differences
between the heat source (e.g. exhaust gases, lubricating oil)
and the receiver streams.
From a thermal perspective, the existence of energy and exergy
flow rates potentially available for
recovery suggests that there is potential for improving the
system’s efficiency and therefore
reducing fuel consumption. However, the fact that most of the
waste heat is available during sea
passages, when on board heat demand is already fully fulfilled
by the use of waste heat from the
main and auxiliary engines, suggests that improvements would
require more complex technical
arrangements.
The utilisation of heat-to-power technologies represents one
possible solution for making use of the
waste heat available during sea passages. This possibility was
explored by Ahlgren et al. [25] and
showed significant potential for improving vessel performance.
The use of WHR systems on board
for heat-to-power conversion could also justify efforts in the
improvement of the heat exchanger
network in order to minimize exergy destruction and, therefore,
allowing additional exergy to be
recovered to useful power for on board use. This additional
effort would be particularly justified in
the case of the lubricating oil cooler and the jacket water
cooler, where the exergy destruction
happens at a higher rate therefore suggesting that the highest
potential for improvement is located.
The use of thermal energy storage devices could constitute an
alternative solution for reducing fuel
consumption by providing a buffer between the excess energy
available during sea passages and the
unfulfilled demand during port stays. A dedicated study, as
proposed by the authors in the case of a
product tanker [26] is required for providing an estimate of the
potential for recovery and of the
required thermal storage capacity.
-
4.2 Limitations and further work
The limited amount of data, both from measurement and design
perspective, limits parts of the
analysis and therefore prevents to dig further into certain
parts of the ship energy systems. The
absence of measurements of the temperature levels of the heat
demand in different parts of the ship
prevents further considerations on heat i ntegration. It is
likely that a number of users on board require low-grade it, as in
the case of HVAC
pre-heaters and re-heaters, which could be provided by
recovering heat from low-grade heat sources
such as the lubricating oil cooler. In addition, strong
assumptions were required for the calculation
of the auxiliary heat demand on board and a significant
improvement in the reliability of the results
could be achieved if additional measurements were available, in
particular for instantaneous fuel
consumption for the ABs and for the heat flow recovered in the
HT cooling systems.
A similar discussion can be presented for on board electrical
users. The absence of measurements
makes it impossible to draw conclusion on possible design and
operational savings related to a
minimized consumption. This influenced the possibility to
analyse the operative efficiency of a
number of systems and components, particularly HVAC, cooling
systems and engine room
ventilation, which not only are expected to contribute
extensively to on board energy demand, but
that are also often related to important improvement potential.
Most of the cooling pumps on board
are in fact equipped with frequency controllers, whose
efficiency in the reduction of pump power
demand was however impossible to determine.
5. Conclusion The results of this study showed that the system
under analysis was designed with significant efforts
for improving energy efficiency; however, many parts of the
system could be improved in order to
reduce fuel consumption.
The main potential proved to come from the main engines, which
are most often operated at low load and, therefore, at low
efficiency. This situation could be improved by engine de-
rating or by a hybridization of the system.
Exergy losses, mostly in the exhaust gas, also provide potential
for improvement. Waste heat recovery through heat-to-power
technologies is a viable option for the system under
study, although a large part of the heat is already recovered
for on board heat demand.
Alternatively, the unbalance between heat availability and
demand during sea passages and port stays could be solved through
the use of a thermal energy storage system, which would
lead to a reduction in the amount of fuel needed by the
auxiliary boilers which today
accounts for 7% of total the yearly fuel consumption.
The results generated by the energy and exergy analysis applied
to the case study constitute a
starting point for future work related to the improvement of the
existing systems, as well as for the
design of new similar ships. This paper partly fills the
existing gap in literature concerning the
analysis of ship operation in terms of energy use, with
particular reference to cruise ships.
Acknowledgments The authors would like to thank the Swedish
Energy Agency and the Swedish Maritime
Administration for financing the main authors of this work. The
authors would also like to thank
Rederiaktiebolaget Eckerö for contributing with data and
information regarding the case study
vessel and the crew of M/S Birka Stockholm for their help and
support during the visits for data
gathering.
Appendix A
-
Table A1: Summary of the assumptions employed in the processing
of measured values for ship
energy and exergy systems analysis.
Variable Equation
, ,air comp outT
1
, ,
, ,
,
( 1)
k
k
air comp in
air comp in
TC is
TT
,is comp P2(load)
airm ,max
60*2
cyl air cyl
vol
V nN
,fuel MEm , , 0 1,
MEfuel ME des norm
ME des
nm a a frp
n
bsfcME P2(loadME)
bsfcAE P2(loadAE)
MEW ,63.6
fuel ME
ME
m
bsfc
propW 0.98 MEW [27]
egm air fuelm m
vol ,
,
[K]
51313 [ ]
6
air inc
ocair in
Tr
rT C
[28]
air , ,
,CAC,out
air CAC out
air air
p
R T
,eg turbm , , , , ,
mech, , , , , ,
( )
( )
p air air comp out air comp in
air
TC p eg eg turb in eg turb out
c T Tm
c T T
,eg bypassm ,eg eg turbm m
coolingQ ,fuel air in eg outQ Q Q W
LTQ 2,
2, 2,
( )
( ) ( )
LT ME
cooling
LT ME HT ME
P loadQ
P load P load
HTQ cooling LTQ Q
,w HTm , ,w HT des MEm load
,w LTm ,L ,w T des MEm load
,LOwm , ,w LO des MEm load
,CAC HTQ 2( )CAC MEQ P load
,CAC LTQ ,CAC CAC HTQ Q
JWQ ,HT CAC HTQ Q
LOQ ,LT CAC LTQ Q
-
Table A2. Summary of the energy and exergy flow rates
represented in the Sankey and Grassmann
diagrams, referred to 11 months of ship operations. Values are
provided in TJ
Type From To Energy flow rate Exergy flow rate
CH Fuel Main engines 203,5 216,6
CH Fuel Auxiliary engines 111,8 119,2
CH Fuel Auxiliary boilers 17,3 18,4
M Main engines Gearbox 75,2 75,2
H Main engines Charge air cooler (ME) 10,1 1,1
H Main engines HRSG (ME) 68,7 27,9
H Main engines Lubricating oil cooler (ME) 34,4 6,3
H Main engines Jacket water cooler (ME) 18,5 6,1
H Charge air cooler (ME) HT cooling systems 2,0 0,4
H Charge air cooler (ME) LT cooling systems 8,2 0,7
H Jacket water cooler (ME) HT cooling systems 18,5 4,2
H Lubricating oil cooler (ME) LT cooling systems 34,4 5,2
H HRSG (ME) Heat distribution system 15,8 5,4
H HRSG (ME) Environment 52,9 19,6
EL Auxiliary engines Switchboard 45,7 45,7
H Auxiliary engines Charge air cooler (AE) 5,0 1,0
H Auxiliary engines HRSG (AE) 39,5 16,0
H Auxiliary engines Lubricating oil cooler (AE) 14,2 2,7
H Auxiliary engines Jacket water cooler (AE) 9,5 3,1
H Charge air cooler (AE) HT cooling systems 0,3 0,1
H Charge air cooler (AE) LT cooling systems 4,7 0,5
H Jacket water cooler (AE) HT cooling systems 9,5 2,1
H Lubricating oil cooler (AE) LT cooling systems 14,2 1,9
H HRSG (AE) Heat distribution system 13,4 7,3
H HRSG (AE) Environment 26,1 8,7
M Gearbox Propeller 73,7 73,7
EL Switchboard Thrusters 0,6 0,6
EL Switchboard Other el. users 45,1 45,1
H Heat distribution system Fuel heating 3,2 1,0
H Heat distribution system Other heat users 58 17,6
H HT cooling systems LT cooling systems 14,0 2.8
H HT cooling systems Heat distribution system 16,3 3.3
H LT cooling systems Environment 75,6 0,9
H Gearbox Environment 1,5 1,5
Nomenclature Letter symbols
B exergy, J
B exergy flow, W
bsfc break specific fuel consumption, g/kWh
c specific heat, J/kg K
E energy, J
E energy flow, W
frp fuel rack position
-
h specific enthalpy, J/kg
I irreversibility rate, W
k specific heat ratio
m mass, kg
m mass flow, kg/s
n rotational speed, rpm
Ncyl number of cylinders
p pressure
Pn polynomial of order n
Q heat flow, W
s specific entropy, J/(kg K)
genS entropy generation rate, W/K
T temperature, K or oC
V volume, m3
V volume flow, m3/s
Acronymes
AB auxiliary boiler
AE auxiliary engine
CAC charge air cooler
GDP gross domestic product
HRSG heat recovery steam generator
HRHT heat recovery from the high temperature cooling systems
HT high temperature cooling systems
HVAC heat, ventilation and air conditioning
IMO international maritime organization
JW jacket water
LHV lower heating value, MJ/kg
LO lubricating oil
LT low temperature cooling systems
ME main engine
OECD organisation for economic co-operation and development
(OECD)
SCR selective catalytic reactor
SG shaft generator
SW sea water
WHR waste heat recovery
Greek letters
β compression ratio
λ irreversibility ratio
δ irreversibility share
t exergy efficiency
η energy efficiency
ρ density, kg/m3
Δ finite difference
-
Subscripts
c cold
comp compressor
des design
eg exhaust gas
h hot
i component
in inlet flow
is isentropic
out output flow
prop propeller
tot total
turb turbine
0 reference state
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